1. Field of the Invention
This invention relates to an uninterruptible ac power supply, and more particularly to an uninterruptible ac power supply which, in case of a failure in a commercial ac line or a heavy drop in voltage of the power supplied in the commercial ac line, converts the energy of a battery by a power inverter into ac power and feeds the ac power to a load without any transient or an interruption.
2. Description of the Prior Art
Most, if not all, of the recent devices for communication and data processing do not tolerate even momentary power failure and call for power supplies of extremely high reliability. In this respect, uninterruptible ac power supplies have found widespread acceptance.
FIG. 1, FIG. 2, and FIG. 3 are block diagrams schematically illustrating configurations of conventional uninterruptible ac power supplies. In these diagrams, like symbols denote like or equivalent parts.
In the uninterruptible ac power supply of FIG. 1, a battery charger 2 rectifies the commercial ac and charges a battery 4 while a commercial ac line 1 is normally operating. At the same time, the aforementioned commercial ac line 1 actuates an inverter 3 through the medium of a control unit 10.
The ac power which has been converted to stated voltage and frequency is fed through a switch 6 to a load 7. In this while, a switch 5 is kept open.
When the inverter 3 goes out of order, the switch 6 is opened and the switch 5 is closed. Consequently, the ac power is fed directly from the commercial ac line 1 to the load 7.
When the commercial ac line 1 encounters a failure, the charging of the battery 4 by the battery charger 2 ceases to exist. Since the operation of the inverter 3 is continued by virtue of the energy stored in the battery 4, however, the supply of the ac power to the load 7 proceeds without any transient.
This uninterruptible ac power supply nevertheless suffers from the following drawbacks.
(1) Since the dual power transformations of conversion and inversion are effected by the operation of the charger 2 and the inverter 3 for the purpose of ensuring continuous supply of power to the load 7 even while the commercial ac line is operating normally, the apparatus has very poor efficiency.
(2) Since the power is supplied to the inverter 3 while the battery is kept charged, the apparatus inevitably necessitates use of a battery charger 2 of a large capacity.
(3) The reliability of the power supply is impaired because the battery charger 2 and the inverter 3 are serially connected relative to the load 7. When either of these components encounters a trouble, the load 7 is supplied the power directly by the commercial ac line 1 which is not always guaranteed to provide constant voltage while the component in trouble is being repaired or replaced with a standby.
(4) Any effort to overcome the drawback indicated in the above item (3) and warrant high reliability of the power supply necessitate redundant design such as standby sets of a battery charger and an inverter installed for parallel operation to the regular set.
FIG. 2 represents a configuration of the uninterruptible ac power supply which additionally incorporates a three-winding transformer 9 adapted to ensure stable power supply to the load by the composition of the powers from the commercial ac line and the inverter.
The three-winding transformer 9 has a core 9F divided into three sections with two magnetic shunts 9S and has a third winding 9C (the output winding connected to the load 7) formed on the central section and a first winding 9A (the input winding for the commercial ac line) and a second winding 9B (the winding for the inverter) formed respectively on the opposite terminal sections.
In FIG. 2, while the commercial ac line 1 is operating normally, the power from this line is transferred through the path of the switch 5.fwdarw.the first winding 9A of the three-winding transformer 9.fwdarw.the third winding 9C of the same transformer 9.fwdarw.the voltage stabilizing circuit 8 to the load 7.
In contrast, the power from the inverter 3 is transferred through the path of the battery charger 2.fwdarw.the battery 4.fwdarw.the inverter 3.fwdarw.the switch 6.fwdarw.the second winding 9B of the three-winding transformer 9.fwdarw.the third winding 9C of the same transformer.fwdarw.the voltage stabilizing circuit 8 to the load 7.
In this case, the flow of power from the inverter 3 to the load is nulled and the inverter is set to its standby status by controlling the output phase of the inverter 3 to keep an in-phase relation with the voltage phase in the load 7.
When the commercial ac line 1 encounters a power failure, the power stored in the battery 4 is immediately transformed to an ac power in the aforementioned path going through the inverter 3 and supplied in the form of ac to the load.
This configuration materializes a highly efficient, highly reliable and economical uninterruptible ac power supply free from the drawbacks of the apparatus of FIG. 1 described above.
Even in this apparatus, a review of its operation in terms of the power flow and the phase control within the three-winding transformer 9 reveals that at the instant of a power failure in the commercial ac line, there inevitably occurs a transient of some degree in the output voltage. As a measure to cope with this phenomenon, therefore, the commercial ac line must be given quite rigid voltage monitoring and the inverter must be given rapid phase control.
FIG. 3 is a block diagram illustrating another conventional configuration using a three-winding transformer.
In the apparatus of FIG. 3, while the commercial ac line is operating normally, the power therefrom is transferred through the path of the switch 5.fwdarw.the first winding 9A of the input transformer 9.fwdarw.the third winding 9C of the same transformer.fwdarw.the voltage stabilizing circuit 8 to the load 7.
In the meantime, part of the power from the commercial ac line is separated at the first winding 9A of the input transformer 9 and transferred through the path of the second winding 9B.fwdarw.a reversible power converter 12 (in the mode of conversion).fwdarw.the battery 4, there to charge the battery 4.
A control unit 10 is possessed of a switch function for enabling the reversible power converter 12 to be operated in the conversion mode when the commercial ac line 1 is in its normal status and in the inversion mode when the commercial ac line is interrupted. It further possesses functions to effect automatic synchronization, voltage control and switching.
Owing to the function to effect automatic synchronization, the control unit 10 keeps the frequency of an oscillator (not shown) synchronized with that of the commercial ac line while the commercial ac line is normally operating and, upon detection of an interruption in the commercial ac line, immediately opens the switch 5 and issues a drive signal to the reversible power converter 12 to operate it on the inversion mode and supply the power through the second and third windings 9B, 9C of the transformer, the iron core 9F, and the voltage stabilizing circuit 8 to the load 7.
When the commercial ac line 1 resumes its normal status and the phase of the commercial ac line 1 and that of the output voltage of the reversible power converter 12 being operated in the inversion mode are synchronized, the aforementioned control unit 10 stops the driving of the aforementioned reversible power converter 12 and, at the same time, closes the switch 5 to change the source for the power of the load 7 to the commercial ac line 1 from the battery 4.
At the same time, the control unit 10, by its switching function, further causes the reversible power converter 12 to operate in the conversion mode and control the output voltage thereof to charge the battery 4.
As is evident from the foregoing desdription, in the apparatus of FIG. 3, the energy of the battery 4 is immediately transferred through the path of the reversible power converter 12 (in the inversion mode)--the second winding 9B of the transformer 9--the third winding 9C of the transformer--the voltage stabilizing circuit 8 to the load 7 on occurrence of a power failure in the commercial ac line 1.
This apparatus has a salient characteristic that since a converter circuit and an inverter circuit are combined integrally as the reversible power converter 12 and share the use of the second winding 9B of the transformer 9, no use is found for an independent battery charger. In spite of this feature, the apparatus suffers from the following drawbacks.
(1) The commercial ac line 1 must be kept under constant watch against occurrence and termination of power failure. On detection of such occurrence or termination of the power failure, the reversible power converter 12 must be switched by proper switch means to the inversion mode or to the conversion mode.
(2) Since the first winding 9A and the second winding 9B of the transformer 9 are tightly coupled by the same section of iron core 9F, no parallel operation can be effected between the commercial ac line 1 and the reversible power converter 12 in the inversion mode. As the result, the control circuit for permitting high-speed switching of mode becomes complicated and the occurrence of transient phenomenon of output voltage during this switching can not be substantially precluded.